This invention relates to an image forming apparatus, and more particularly to an image forming apparatus having sheet feeding condition detecting means for detecting sheet feeding conditions of various operations for a sheet feeding process such as a sheet leading operation to a sheet feed path, a sheet feeding operation along the sheet path, a sheet residue detecting opration in the sheet feeding process to perform a full-color copying operation without a sheet feed error and an imaging error.
An image forming apparatus for forming a full-color image on a photosensitive recording medium such as a microcapsule sheet is generally constructed to operate as follows: An original is sandwiched between an original stand cover and an original stand glass which are reciprocally linearly movable on a top portion of the image forming apparatus. The original is illuminated with light emitted from a light source which comprises a halogen lamp and a semicylindrical reflecting mirror, and the original stand cover and the original stand glass are moved, thereby scanning the original with the light. The light which has scanned the original is led through optical systems to an exposure stand that is located centrally in the image forming apparatus. The light is irradiated onto one surface of an elongated band-like microcapsule sheet supplied to the exposure stand, so that a latent image is formed on the microcapsule sheet. The microcapsule sheet having the latent image formed thereon is then pressed against a developer sheet in a pressure-developing unit, thereby to form a visible image on the developer sheet. The visible image formed on the developer sheet is then fixed thereto by a heat-fixing unit, after which the developer sheet is discharged from the image forming apparatus.
In the above conventional image forming apparatus, when the residual quantity of the microcapsule sheet wound around a cartridge shaft for supplying the microcapsule sheet to the exposure stand, the pressure-developing unit and other elements for performing a series of image forming processes becomes zero in a sheet feeding process, only a developer sheet having a developer agent thereon would be supplied to the pressure-developing unit, and therefore the developer agent coated on the developer sheet is directly contacted with a pressure roller of the pressure-develping unit. Accordingly, the following disadvantages may occur in the conventional image forming apparatus: the pressure roller is soiled by the developer sheet, and a sheet jam of the developer sheet occurs because the tension applied to the microcapsule sheet is lost due to separation of the trailing end portion of the microcapsule sheet from the cartridge shaft and the developer sheet is not forcedly guided to an inlet of the pressure-developing unit by the microcapsule sheet. Therefore, it is required to cease a pressure-developing process and wind up the unexposed residual microcapsule sheet at the time when the residual quantity of the microcapsule sheet wound around the cartridge shaft becomes zero. However, when the pressure-developing process is ceased halfway, it more frequently occurs that only a part of the original image is formed on the developer sheet, that is, a complete copied image can not be formed on the microcapsule sheet, and this is a waste of the developer sheet.
Further, in the above conventional image forming apparatus, when there is no residual microcapsule sheet wound around the cartridge shaft, it is necessary to exchange an used cassette having no microcapsule sheet for a new one having an unused (unexposed) microcapsule sheet and exchange the take-up shaft for winding up the used (exposed) microcapsule sheet is also exchanged for a new one. In this case, it is required to carry out a sheet feeding operation in which the leading end of an unused microcapsule sheet is lead to a sheet feed path and fed through a sheet feed path to wind up the leading end around the new take-up shaft. This winding operation of the leading end of the microcapsule sheet around the take-up shaft is performed as follows.
The leading end of the microcapsule sheet is automatically led through the sheet feed path provided inside of the image forming apparatus, and then is automatically fed along the sheet feed path by a predetermined amount (distance) required for winding the leading end around the take-up shaft. During the above winding operation, the operator observes the sheet feed condition to check for sheet feed error, that is, the sheet feed error is detected by the operator's observing the feed. When the sheet feed error is observed by the operator, the microcapsule sheet is removed from the sheet feed path of the image forming apparatus, and then the sheet feeding operation is resumed.
As described above, in the conventional image forming apparatus, the sheet feed error is detected after the microcapsule sheet is fed through the sheet feed path by the predetermined amount (distance) required for winding the microcapsule sheet around the take-up shaft. However, in this sheet feed error detection manner as described above, if the microcapsule sheet erroneously wanders into a different feed path from the predetermined sheet feed path during the sheet feeding operation or there occurs a sheet feed error in which the microcapsule sheet is wound up by another roller, it requires more time and labor for restoring the troubled microcapsule sheet because such an error is not detected at an early stage of the sheet feeding operation. Further, when there occurs an error in which the leading end of the microcapsule sheet is not drawn out of the cassette, the error is not detected until the time which is required for winding the microcapsule sheet around the take-up shaft has elapsed. This is a waste of time as well as troublesome.